Many specialized terms and abbreviations are used in the communications arts. At least some of the following are referred to within the text that follows, such as in this background and/or the description sections. Thus, the following terms and abbreviations are herewith defined:
3GPP3rd Generation Partnership ProjectCDMACode Division Multiple AccessCL1Closed Loop mode 1CQIChannel Quality IndicatorD-TxAADual Transmit Adaptive ArrayG-RakeGeneralized RakeMIMOMultiple Input, Multiple OutputMISOMultiple Input, Single OutputMLMaximum LikelihoodMMSEMinimum Mean Squared ErrorMSMobile StationRBSRadio Base StationSIMOSingle Input, Multiple OutputSINRSignal-to-Interference-plus-Noise RatioSISOSingle Input, Single OutputSNRSignal-to-Noise RatioSTTDSpace Time Transmit DiversityTTITransmission Time IntervalWCDMAWideband CDMAZFZero Forcing
Electronic communication forms the backbone of today's information-oriented society. Electronic communications are transmitted over wireless or wired channels using electromagnetic radiation. The availability and capacity of electronic communications is typically limited by the bandwidth of the communications channel. Especially in wireless environments, the bandwidth of a communications channel may be limited by the finite nature of the electromagnetic spectrum.
The available bandwidth of a communications channel, even given a finite allocation of the electromagnetic spectrum, may be increased by adopting any of a number of different schemes. These schemes enable more information to be communicated in a given spectrum allocation. Efficient utilization of spectrum can reduce the cost of communication services being provided, can enable richer communication services to be provided, or both.
Example schemes include compression of the information, dense symbol modulations, error correction encoding, utilizing multipath signal diversity, adopting multiple antennas at the transmitter and/or receiver, directing transmissions and/or receptions with beamforming, and so forth. Many of these schemes may be combined into the same communication system and used together to further increase the efficient utilization of spectrum.
For example, a Rake receiver can collect signal energy from diverse signals to strengthen reception and demodulation of the actual information. A conventional Rake receiver can use multipath reception to improve the SNR of a received multipath signal by combining the paths with appropriate weights. The Rake receiver includes fingers that are placed on the diverse signal paths. Received traffic symbols from each signal path are despread for each finger in accordance with known spread spectrum technologies. The Rake receiver combines the despread traffic symbol values as received on the diverse paths into an estimated symbol using combining weights. The estimated symbols resulting from weighted combinations tend to have a lower error rate as compared to those resulting from single-path reception. However, the conventional Rake receiver does not address interference, which can result in degraded receiver performance.
In contrast, a Generalized Rake, or G-Rake, receiver can suppress interference. A G-Rake receiver may place fingers at other locations in addition to those for signal paths to enhance overall signal reception and symbol demodulation. More specifically, interference may be suppressed by using a set of combining weights that account for the effect of noise and interference to thereby improve the Signal-to-Interference-plus-Noise Ratio (SINR) of a received multipath signal. Both conventional Rake and G-Rake receivers may operate in SISO, SIMO, MISO, and MIMO scenarios.
With MIMO scenarios, multiple antennas are utilized at the transmitter and at the receiver. A MIMO communication scenario involves N transmit antennas and M receive antennas. The variables N and M are positive integers greater than one; they may be the same or different integers. The transmit and/or receive processing is designed in a manner so as to improve bit/block error rates as compared to communication scenarios that have a single transmit antenna and/or a single receive antenna.
A 2×2 MIMO scenario (e.g., a scenario with 2 transmit and 2 receive antennas) has been standardized within 3GPP. This 3GPP 2×2 MIMO scenario is based upon the so-called Dual-Transmit Adaptive Array (D-TxAA) concept. D-TxAA is an extension of a previously-standardized transmit diversity scheme known as Closed Loop mode 1 (CL 1). D-TxAA has two modes of operation: single stream mode and dual stream mode. In either mode, the transmitter operations are specified (e.g., those operations of an RBS in a WCDMA system). Although performance requirements for the receiver are specified, the specific operations for the receiver structure are not stipulated.
There are existing linear interference suppression approaches for MIMO. Two examples are reviewed below: the parametric G-Rake receiver and the MIMO chip equalizer. For parametric G-Rake linear interference suppression, such receivers operate on the symbol level. For a 2×2 MIMO scenario, this receiver computes an impairment covariance matrix Ru via equation (1):Ru=α1R11+α2R22+α12+R12++jα12−R12−+βRn,  (1)where R11 captures the interference due to transmit antenna 1, R22 captures the interference due to transmit antenna 2, R12+ and R12− represent cross-antenna interference, and Rn accounts for white noise passing through the receive filter.
The α and β weighting terms for this model-based parametric G-Rake receiver are given by equations (2):
                                                        α              1                        =                                          1                                                                            γ                      p                                        ⁡                                          (                      1                      )                                                        ⁢                                      N                    p                                                              [                                                                    Γ                                          D                      /                      P                                                        ⁡                                      (                                                                                                                        γ                            s                                                    ⁡                                                      (                            1                            )                                                                          ⁢                                                                                                                                        b                              11                                                                                                            2                                                                    +                                                                                                    γ                            s                                                    ⁡                                                      (                            2                            )                                                                          ⁢                                                                                                                                        b                              12                                                                                                            2                                                                                      )                                                  +                                                      Γ                                          V                      /                      P                                                        ⁢                                                            ∑                                              i                        =                        1                                                                    K                        v                                                              ⁢                                                                                            γ                          v                                                ⁡                                                  (                          i                          )                                                                    ⁢                                                                                                                              v                                                          1                              ⁢                              i                                                                                                                                2                                                                                            +                                                                            γ                      o                                        ⁡                                          (                      1                      )                                                        ⁢                                      Γ                                          O                      /                      P                                                                      +                                                      γ                    p                                    ⁡                                      (                    1                    )                                                              ]                                ⁢                                          ⁢                                    α              2                        =                                          1                                                                            γ                      p                                        ⁡                                          (                      2                      )                                                        ⁢                                      N                    p                                                              ⁡                              [                                                                            Γ                                              D                        /                        P                                                              ⁡                                          (                                                                                                                                  γ                              s                                                        ⁡                                                          (                              1                              )                                                                                ⁢                                                                                                                                                  b                                21                                                                                                                    2                                                                          +                                                                                                            γ                              s                                                        ⁡                                                          (                              2                              )                                                                                ⁢                                                                                                                                                  b                                22                                                                                                                    2                                                                                              )                                                        +                                                            Γ                                              V                        /                        P                                                              ⁢                                                                  ∑                                                  i                          =                          1                                                                          K                          v                                                                    ⁢                                                                                                    γ                            v                                                    ⁡                                                      (                            i                            )                                                                          ⁢                                                                                                                                        v                                                              2                                ⁢                                i                                                                                                                                          2                                                                                                      +                                                                                    γ                        o                                            ⁡                                              (                        2                        )                                                              ⁢                                          Γ                                              O                        /                        P                                                                              +                                                            γ                      p                                        ⁡                                          (                      2                      )                                                                      ]                                              ⁢                                          ⁢                      α            12            +                    =                                    1                                                N                  p                                ⁢                                                                                                    γ                        p                                            ⁡                                              (                        1                        )                                                              ⁢                                                                  γ                        p                                            ⁡                                              (                        2                        )                                                                                                                  ⁡                          [                                                                    Γ                                          D                      /                      P                                                        ⁡                                      (                                                                                                                        γ                            s                                                    ⁡                                                      (                            1                            )                                                                          ⁢                                                  Re                          ⁡                                                      [                                                                                          b                                11                                                            ⁢                                                              b                                21                                *                                                                                      ]                                                                                              +                                                                                                    γ                            s                                                    ⁡                                                      (                            2                            )                                                                          ⁢                                                  Re                          ⁡                                                      [                                                                                          b                                12                                                            ⁢                                                              b                                22                                *                                                                                      ]                                                                                                                )                                                  +                                                      Γ                                          V                      /                      P                                                        ⁢                                                            ∑                                              i                        =                        1                                                                    K                        v                                                              ⁢                                                                                            γ                          v                                                ⁡                                                  (                          i                          )                                                                    ⁢                                              Re                        ⁡                                                  [                                                                                    v                                                              1                                ⁢                                i                                                                                      ⁢                                                          v                                                              2                                ⁢                                i                                                            *                                                                                ]                                                                                                                                ]                                      ⁢                                  ⁢                              α            12            -                    =                                    1                                                N                  p                                ⁢                                                                                                    γ                        p                                            ⁡                                              (                        1                        )                                                              ⁢                                                                  γ                        p                                            ⁡                                              (                        2                        )                                                                                                                  ⁡                          [                                                                    Γ                                          D                      /                      P                                                        ⁡                                      (                                                                                                                        γ                            s                                                    ⁡                                                      (                            1                            )                                                                          ⁢                                                  Im                          ⁡                                                      [                                                                                          b                                11                                                            ⁢                                                              b                                21                                *                                                                                      ]                                                                                              +                                                                                                    γ                            s                                                    ⁡                                                      (                            2                            )                                                                          ⁢                                                  Im                          ⁡                                                      [                                                                                          b                                12                                                            ⁢                                                              b                                22                                *                                                                                      ]                                                                                                                )                                                  +                                                      Γ                                          V                      /                      P                                                        ⁢                                                            ∑                                              i                        =                        1                                                                    K                        v                                                              ⁢                                                                                            γ                          v                                                ⁡                                                  (                          i                          )                                                                    ⁢                                              Im                        ⁡                                                  [                                                                                    v                                                              1                                ⁢                                i                                                                                      ⁢                                                          v                                                              2                                ⁢                                i                                                            *                                                                                ]                                                                                                                                ]                                                          (        2        )                                β        =                              N            0                    .                                                Here, Np is the pilot code spreading factor; γs(k) is the fraction of the total base station chip energy allocated to voice (x=v), data (x=s), overhead (x=o), or pilots (x=p) for antenna/stream k; Γx/P is the ratio of chip energies Ex/Ep; bij is the i,j element of pre-coding matrix B; and vij is the ith element of the pre-coding vector v for the jth voice user. It should be noted that in WCDM, for example, the columns of B and the pre-coding vectors v are drawn from the codebook Φ={φ1,φ2,φ3,φ4}, where φk=[1 ej(2k−1)π/4]T. The columns of pre-coding matrix B are selected from orthogonal pairs of Φ in dual stream mode. In single stream mode, on the other hand, one pre-coding vector is chosen for the first column of B while the second column is set to the zero vector ([0 0]T).
A G-Rake receiver utilizes an impairment covariance matrix Ru and net channel estimates to compute combining weights w. The combining weights w for this receiver structure depend on whether one or two streams are being transmitted. For single stream mode, the combining weights wsingle are obtained by solving the following system of equations (3):Ruwsingle=heff(b),  (3)where the notation heff(b) indicates the “effective” net channel coefficients that depend on the pre-coding vector b. The pre-coding vector b is for the single stream mode and is the first column of pre-coding matrix B.
For dual stream mode, two sets of combining weights (w1dual,w2dual) are computed. These weights are obtained by solving the following systems of equations (4):
                                                        (                                                R                  u                                +                                                                            α                      PC                                        ⁡                                          (                      2                      )                                                        ⁢                                                            h                      eff                                        ⁡                                          (                                              b                        2                                            )                                                        ⁢                                                            h                      eff                      H                                        ⁡                                          (                                              b                        2                                            )                                                                                  )                        ⁢                          w              1              dual                                =                                    h              eff                        ⁡                          (                              b                1                            )                                      ⁢                                  ⁢                                            (                                                R                  u                                +                                                                            α                      PC                                        ⁡                                          (                      1                      )                                                        ⁢                                                            h                      eff                                        ⁡                                          (                                              b                        1                                            )                                                        ⁢                                                            h                      eff                      H                                        ⁡                                          (                                              b                        1                                            )                                                                                  )                        ⁢                          w              2              dual                                =                                                    h                eff                            ⁡                              (                                  b                  2                                )                                      .                                              (        4        )            Here, the heff(bn) is the effective net channel coefficient vector due to pre-coding for stream n, and αPC(n) is a per-code scaling factor that multiplies the outer product of the effective net channel coefficients to account for the fact that the same channelization codes are used for both streams (i.e., code reuse interference). The pre-coding vector b1 corresponds to the first column of pre-coding matrix B, and the pre-coding vector b2 corresponds to the second column of pre-coding matrix B.
With parametric G-Rake linear interference suppression, symbol estimates for either the single or the dual stream mode are obtained by computing the inner product of the combining weights for the given stream with the despread traffic symbol values. It should be noted that the G-Rake receiver described immediately above can be simplified to reduce its complexity. The reduced complexity version involves the estimation of fewer matrix terms and therefore fewer scaling parameters.
For MIMO chip equalizer linear interference suppression, such receivers are extensions of the chip equalizer proposed for interference suppression with multiple receive antennas (e.g., a so-called type 3 receiver) for the case of dual stream transmissions. The receiver description that follows is taken from a 3GPP working group document (RAN4, document number R4-061370, 2006). Chip equalizers employ a different sequence of operations than a G-Rake receiver. Generally, a G-Rake receiver first despreads the chip sequence across multiple respective fingers to produce respective despread symbol values and then combines the resulting despread values to form symbol estimates. In contrast, a chip equalizer essentially combines first and then despreads the signal with a single finger to form the symbol estimates.
With a MIMO chip equalizer, it is given that the chip level signal from stream 1 and stream 2 are s1 and s2, respectively. The received signal is represented by equation (5):
                                                        r              =                                                H                  ⁢                                      B                    ~                                    ⁢                  s                                +                n                                                                          where                                                                              s                =                                  [                                                                                                              s                          1                                                                                                                                                              s                          2                                                                                                      ]                                            ,                                                                                          H                =                                  [                                                                                                              H                                                      1                            ,                            1                                                                                                                                                H                                                      1                            ,                            2                                                                                                                                                                                        H                                                      2                            ,                            1                                                                                                                                                H                                                      2                            ,                            2                                                                                                                                ]                                            ,                                                            and                                                                              B                ~                            =                                                [                                                                                                                                          b                            11                                                    ⁢                          I                                                                                                                                                  b                            12                                                    ⁢                          I                                                                                                                                                                                          b                            21                                                    ⁢                          I                                                                                                                                                  b                            22                                                    ⁢                          I                                                                                                      ]                                .                                                                        (        5        )            
The signal {tilde over (B)}s is the transmitted signal after pre-coding which propagates through the channel matrix H. The other cell noise is denoted by n, and it is assumed to be white and Gaussian. The assumption on transmissions by other users is that they are using the same beam coefficients as the user of interest. Of course, this creates a mismatch between the true transmission and the derived equalizer coefficients.
The channel matrix from transmit antenna j to receive antenna i models the channel convolution. Thus, the channel matrix H is represented by equations (6) and (7) as follows:
                              H                      i            ,            j                          =                                                       [                                                          ⁢                                                                                          h                                                                        N                          s                                                ×                                                  (                                                      F                            +                            L                            -                            1                                                    )                                                                                            i                        ,                        j                                                                                                                        0                                                                        N                          s                                                ×                        1                                                                                                  Λ                                                                              0                                                                        N                          s                                                ×                        1                                                                                                                                                        0                                                                        N                          s                                                ×                        1                                                                                                                        h                                                                        N                          s                                                ×                                                  (                                                      F                            +                            L                            -                            1                                                    )                                                                                            i                        ,                        j                                                                                                  Λ                                                                              0                                                                        N                          s                                                ×                        1                                                                                                                                                        0                                                                        N                          s                                                ×                        1                                                                                                                        0                                                                        N                          s                                                ×                        1                                                                                                  O                                                                              0                                                                        N                          s                                                ×                        1                                                                                                                                                        0                                                                        N                          s                                                ×                        1                                                                                                                        0                                                                        N                          s                                                ×                        1                                                                                                  Λ                                                                              h                                                                        N                          s                                                ×                                                  (                                                      F                            +                            L                            -                            1                                                    )                                                                                            i                        ,                        j                                                                                                        ⁢                                                          ]                        ⁢                                                  ,                                                  ⁢                                                                      ⁢                                                                    ⁢            with                                              (        6        )                                                          ⁢                              h                                          N                s                            ×                              (                                  F                  +                  L                  -                  1                                )                                                    i              ,              j                                =                                    [                                                                                          h                                              0                        ,                                                  L                          -                          1                                                                                            i                        ,                        j                                                                                                  Λ                                                                              h                                              0                        ,                        0                                                                    i                        ,                        j                                                                                                                                  M                                                        O                                                        M                                                                                                              h                                                                                                    N                            s                                                    -                          1                                                ,                                                  L                          -                          1                                                                                            i                        ,                        j                                                                                                  Λ                                                                              h                                                                        N                          s                                                ,                                                  -                          1                                                ,                        0                                                                    i                        ,                        j                                                                                                        ]                        .                                              (        7        )            
Here, Ns and L are the number of samples per chip and the length of the impulse response, respectively. If the equalizer length in chips equals F, the size of the channel matrix H equals (2Ns,F) ×2(F+L−1).
The equalizer filter coefficients are obtained by first constructing a matrix filter via equation (8):
                                                                        w                Rx                            =                                                                    B                    ~                                    H                                ⁢                                                      H                    H                                    ⁡                                      (                                                                  H                        ⁢                                                  B                          ~                                                ⁢                                                                              B                            ~                                                    H                                                ⁢                                                  H                          H                                                                    +                                              2                        ⁢                                                                                                  ⁢                                                  σ                          n                          2                                                ⁢                        I                                                              )                                                                                                                          =                                                                    B                    ~                                    H                                ⁢                                                                            H                      H                                        ⁡                                          (                                                                        HH                          H                                                +                                                  2                          ⁢                                                                                                          ⁢                                                      σ                            n                            2                                                    ⁢                          I                                                                    )                                                        .                                                                                        (        8        )            An estimate of the chip pair s1(d) and s2(d) can be obtained from rows d and F+L−1+d of the matrix filter wRx. In other words, the chip pair can be estimated as shown in equation (9):
                              [                                                                                                                s                      ~                                        1                                    ⁡                                      (                    d                    )                                                                                                                                                                  s                      ~                                        2                                    ⁡                                      (                    d                    )                                                                                ]                =                              [                                                                                                      w                      Rx                                        ⁡                                          (                      d                      )                                                                                                                                                              w                      Rx                                        ⁡                                          (                                              F                        +                        L                        -                        1                        +                        d                                            )                                                                                            ]                    ⁢                      r            .                                              (        9        )            With such MIMO chip equalizer linear interference suppression, the symbol estimates for the two streams are then obtained by despreading the resulting chip sequences.
Unfortunately, there are deficiencies in the foregoing state of the art for interference suppression with regard to both parametric G-Rake receivers and MIMO chip equalizers. With regard to the parametric G-Rake approach, performance at high SINR levels is problematic. The parametric formulation entails the knowledge of channel delays. At low to moderate SINR, errors in channel delay estimation generally do not affect receiver performance. At high SINR, on the other hand, highly accurate delay estimates are required, and even small errors in channel delay estimation effectively limit the peak throughput of the receiver. This peak throughput limitation can be relatively significant, especially when there is little or no channel coding (i.e., at high data rates). A limitation of peak throughput is considered a drawback to purchasers of communications systems and from a user experience point of view. Although the errors in channel delay estimation can be addressed, doing so involves considerable complexity and cost expenditures.
With regard to the MIMO chip equalizer approach, such a receiver generally performs worse than the inventive embodiments described herein below in terms of either throughput or block error rate. This is caused by the chip equalizer receiver making some explicit assumptions regarding the signals being transmitted by the base station (e.g., by an RBS). Deviations from these assumptions by the transmitted signals can result in poor overall receiver performance. Consequently, there is a need to address these deficiencies in the current state of the art. Such deficiencies and other needs are addressed by one or more of the various embodiments of the present invention.